docs: def initialize_hmac(self):
          # type: (EncryptionMetadata) -> hmac.HMAC
          """Initialize an hmac from a signing key if it exists
          :param EncryptionMetadata self: this
          :rtype: hmac.HMAC or None
          :return: hmac
          """
          if self._signkey is not None:
              return hmac.new(self._signkey, digestmod=hashlib.sha256)
          else:
              return None